The present invention relates to a feed water supply system for a power plant and, more particularly, to a power plant feed water supply system which supplies feed water into a steam generator, using a turbine driven feed water pump.
A conventional feed water supply system for a power plant has a motor driven feed water pump and a turbine driven feed water pump arranged in parallel with each other, which pumps are changed over according to operation conditions, etc. to supply feed water into a steam generator.
For this kind of use, several methods are known for the case of plant starting, for example. One of them is a method in which a motor driven feed water pump is used at an initial stage of starting to supply feed water into a steam generator, and then a turbine driven feed water pump is driven by utilizing extraction-steam from a main steam turbine after steam is generated in the steam generator. Another is a method in which an auxiliary steam source is provided in advance, a turbine driven feed water pump is used from an initial stage of starting to supply feed water into the steam generator, and the turbine driven feed water pump is driven by using extraction steam from a main steam turbine after generation of steam in the steam generator.
Further, a method in which the turbine driven feed water pump is driven with an auxiliary steam source is disclosed in JP A 62-37603 (1987) and JP A 63-99403 (1988), wherein, when an accident occurs on electric transmitting wires and the load of the power plant is reduced to continue operation (so-called, FCB operation), the operation is continued by switching steam to be supplied to a feed water pump turbine from the main turbine extraction steam used at normal operation to auxiliary steam. In particular, in this case, it is suggested that the pressure of the auxiliary steam is changed according to operation conditions.
In the usual power plant, in many cases, two 50% capacity turbine driven feed water pumps and a 25-30% capacity motor driven feed water pump are employed. However, since the motor driven feed water pump is provided inherently as an auxiliary apparatus or a starting apparatus, recently, in many cases, it has been proposed to use a method in which the turbine driven feed water pump is used as much as possible, under ordinary operating conditions, including plant starting and stopping.
However, as to whether or not it is possible to operate only a turbine driven feed water pump under any operational conditions, the answer is no, that is, there are several exceptions. For example, when a boiler is brought into a trip condition due to load interruption, etc., the boiler and turbine metal attain high temperature conditions before the load interruption; however, in case a once through boiler is being used, since the feed water pump for supplying feed water to the boiler is stopped, it must be restarted by using a motor driven feed water pump.
A more detailed explanation will be given. In the case of normal starting, the inner pressure of the boiler is zero or several tens of kg/cm.sup.2, for example, and in order to supply feed water to the boiler by a feed water pump, it is sufficient for the driving steam pressure of the feed water pump turbine to be several kg/cm.sup.2 or so in general, and usually, driving steam of about this pressure value is used. On the contrary, since the inner pressure of the boiler at restarting after a boiler trip remains more than two hundred kg/cm.sup.2 in case of a super critical pressure once through boiler, in order to secure feed water at the restarting using a turbine driven feed water pump, it is necessary for a drive steam pressure of the feed water pump turbine to be several kg/cm.sup.2 to several tens of kg/cm.sup.2 in general.
Here, as the driving steam of the feed water pump turbine, auxiliary steam which is provided in advance or turbine extraction steam is used. The former steam is used at starting and the latter steam is switched during normal operation. Further, operation using the main turbine extraction steam during normal operation is effective from the point of view of an improvement in a plant efficiency, and the extraction steam changes in proportion to the load of the main turbine within a range from 0 kg/cm.sup.2 to several kg/cm.sup.2.
As apparent from the above explanation, in the case of an operation using a boiler feed water pump, including restarting after the boiler trip, it is necessary to provide auxiliary steam at a pressure of several kg/cm.sup.2 to several tens of kg/cm.sup.2 for driving the feed water pump turbine. This causes the turbine efficiency to worsen because, when the steam is changed to the turbine extraction steam, the main turbine extraction steam does not enter the feed water pump turbine, since the auxiliary steam pressure is higher than the main turbine extraction steam pressure, and the operational range of the feed water pump turbine using extraction steam is narrow.
On the contrary, when importance is attached to the plant efficiency and the auxiliary steam pressure is set to a necessary, but minimum, pressure value (usually, 2 kg/cm.sup.2 or so), it is impossible to use the feed water pump at the restarting after boiler trip to supply feed water, and it is necessary to use a motor driven feed water pump.